Method of sensing access positions of arm

1. A method of sensing an access position of an arm that loads or unloads a wafer into or from a wafer cassette with slots for housing wafers, said arm access position sensing method comprising: a calibration step of putting at least two reference wafers in the top and bottom reference slots, optically sensing the positions of these reference wafers, and calculating an offset value inherent to a reference wafer cassette; a mapping step of optically sampling a processing wafer cassette and sensing the presence or absence of a wafer in each slot and information on an apparent thickness (sampling thickness) of said wafer in each of said slots; and the step of calculating an access position of said arm by correcting said sampling thickness on the basis of said offset value.

2. An arm access position sensing method according to claim 1, wherein said calibration step includes the step of causing said arm to come closer to said reference wafer cassette and said wafer until the arm has reached a specific position and sensing information on a reference position (base position) of said arm.

3. An arm access position sensing method according to claim 1, wherein said calibration step includes the step of projecting a light beam onto the edge sides of said two reference wafers housed in said reference cassette, finding the distance between the two shade centers of said light beam, and calculating a reference slot position by dividing the distance by (the number of all slots-1).

4. An arm access position sensing method according to claim 1, wherein said calibration step calculates the central positions (map positions) and apparent thicknesses (map thicknesses) of said reference wafers.

5. An arm access position sensing method according to claim 1, wherein said mapping step includes the step of projecting said light beam onto the edge sides of the wafers housed in said processing wafer cassette, comparing the shading center positions of said light beam related to said wafers with said reference slot positions, and generating an error signal for cross slot when the comparison value has exceeded a specific allowed value.

6. An arm access position sensing method according to claim 4, wherein said mapping step includes the step of calculating the difference between said sampling thickness and said map thickness (map wafer offset value).

7. An arm access position sensing method according to claim 6, wherein said mapping step includes the step of correcting a preset actual wafer thickness on the basis of said map wafer offset value determined for each of said slots and calculating the pitch distance on the basis of information on the corrected wafer thickness.

8. An arm access position sensing method according to claim 7, wherein said pitch distance calculating step includes the step of comparing the calculated distance with a preset allowed value (slot pitch allowed value) and generating an error signal when the calculated distance has exceeded the allowed value.

9. An arm access position sensing method according to claim 7, wherein said mapping step includes the step of generating an error signal when said corrected wafer thickness is smaller than a specific transfer allowable wafer thickness.

10. An arm access position sensing method according to claim 1, wherein said arm access position sensing step includes the step of correcting said reference slot positions determined in said calibration step on the basis of an offset value using said actual wafer thickness as a reference, when it has been judged in said mapping step that none of said wafers are present.

11. An arm access position sensing method according to claim 1, wherein said arm access position sensing step calculates the arm access positions of said arm on the basis of the wafer thickness corrected in said mapping step, when it has been judged in said mapping step that said wafers are present.

12. An arm access position sensing method according to claim 1, wherein said calibration step includes a horizontality adjusting step of adjusting the horizontality of a cassette stage on which said reference wafer cassette is placed so that the horizontality may be in the range of allowed values.

13. An arm access position sensing method according to claim 1, wherein said calibration step includes a horizontality adjusting step of adjusting the horizontality of said arm so that the horizontality may be in the range of allowed values.

14. A method of setting an access position of an arm with respect to a processing wafer cassette in a processing system to which a transfer chamber including a transfer mechanism with an arm for transferring a wafer, a process chamber for subjecting said wafer to specific processes, and a load/unload chamber for housing a processing cassette that holds unprocessed or processed wafers and having an optical sensor for sensing the edge sides of said wafers, are connected, with the process chamber and load/unload chamber being arranged around said transfer chamber, said access position sensing method comprising: an arm horizontal adjusting step of horizontally adjusting the movement of said wafer held by said arm; a stage horizontal adjusting step of horizontally adjusting the mounting surface of a stage on which said processing wafer cassette in said load/unload chamber is placed; an adjusting wafer cassette checking step of checking to see if an adjusting wafer cassette has a A-determined shape (offset); a base position sensing step of putting reference wafers in the top and bottom slots of said adjusting wafer cassette, causing said arm to approach the back of the wafer in said bottom slot, and determining the position at which the arm first comes into contact with the wafer to be a base position; a mapping start position setting step of causing said optical sensor to sense the position of said reference wafer in the top slot of said adjusting wafer cassette and setting the position as a mapping start position; a reference slot position calculating step of causing said optical sensor to sense the positions of said reference wafers in the top slot and bottom slot of said adjusting wafer cassette on said stage and calculating the position of each of the reference slots; a mapping step of putting wafers to be processed in the top slot down to the bottom slot of a processing wafer cassette, placing the cassette on said stage, causing said optical sensor to sense the positions of wafers to be processed, calculating the presence or absence and thickness of wafers to be processed and the position of each slot, judging whether said slot position is within an allowed range predetermined on the basis of said reference slot position, and determining whether or not said wafer cassette can be used; and an arm access position setting step of correcting the positions of the slots in the processing wafer cassette on the basis of the reference information found using said adjusting wafer cassette and setting an access position of the arm that unloads or loads a wafer to be processed from or into said processing cassette.

15. An arm access position sensing method according to claim 14, wherein said arm horizontal adjusting step places a wafer on a pick of said arm and adjusts said arm so that the back of the wafer W may be almost in parallel with the inner bottom surface of a transfer chamber set in a horizontal plane, with a specific distance between them.

16. An arm access position sensing method according to claim 14, wherein said stage horizontal adjusting step makes compensation by tilting said stage according to the inclination, when said wafer has a cutout section, such as an orientation flat section or a notch section, is housed with said cutout section aligning with the back of said adjusting cassette, opposite to its opening, and when the wafer has tilted in a slot.

17. An arm access position sensing method according to claim 14, wherein said adjusting wafer cassette distance A' between said mounting surface and specific point A on the front of the opening of the adjusting wafer cassette at the back of wafer in the bottom slot, distance B' between said mounting surface and specific point B on the back of the adjusting wafer cassette at the back of wafer in the bottom slot, distance C' and distance D' between said mounting surface and point C and point D set in almost symmetric positions on the load/unload side of the adjusting wafer cassette, respectively, with a line connecting said point A to point B running between point C and point D, distance E' between said point C and said point D, distance .alpha.' between said mounting surface and point .alpha. on the wafer in the top slot corresponding to point A on the wafer in the bottom slot, and distance .beta.' between said mounting surface and point .beta. on the wafer in the top slot corresponding to point B on the wafer in the bottom slot, the distances A', B', C', C', E', (A'-B'), (C'-D') of the bottom slot, the distances .alpha.',.beta.' of the top slot, and a pitch distance of {(.alpha.'+.beta.')-(A'+B')}/2 conform to predetermined reference values or lie in predetermined allowed ranges.

18. An arm access position sensing method according to claim 14, wherein said mapping start position setting step sets a mapping start position to the position in which said reference wafer in the top slot of said adjusting wafer cassette has been sensed with optical sensors arranged on a line connecting the front of the opening of said adjusting wafer cassette to its back.

19. An arm access position sensing method according to claim 14, wherein said reference slot position calculating step places said adjusting wafer cassette that houses wafers in the top slot and bottom slot on said stage, moves said adjusting wafer cassette upward, senses the positions of the wafers in the top slot down to the bottom slot, binarizes the positions, and calculates the central position of each wafer, and divides the distance between the calculated two central positions by (the number of slots-1) to find a reference slot position, senses the map position or the central position of a wafer housed in the bottom slot and the map thickness or apparent thickness of the wafer, measures the difference between the map thickness and that of said reference wafer, and calculates a map wafer offset.